System that compensates for coupling based on sensing a neighbor using coupling

ABSTRACT

Shifts in the apparent charge stored on a floating gate (or other charge storing element) of a non-volatile memory cell can occur because of the coupling of an electric field based on the charge stored in adjacent floating gates (or other adjacent charge storing elements). To compensate for this coupling, the read or programming process for a given memory cell can take into account the programmed state of an adjacent memory cell. To determine whether compensation is needed, a process can be performed that includes sensing information about the programmed state of an adjacent memory cell (e.g., on an adjacent bit line or other location).

This application is a divisional application of U.S. patent applicationSer. No. 11/459,000, “System That Compensates For Coupling Based OnSensing A Neighbor Using Coupling,” filed on Jul. 20, 2006, by Yan Li,et al., published on Jan. 24, 2008 as U.S. Publication No. US20080019175A1, Attorney Docket No. SAND-01083US1, incorporated herein byreference.

CROSS-REFERENCE TO RELATED APPLICATIONS

The following applications are cross-referenced and incorporated byreference herein in their entirety:

“System That Compensates For Coupling Based On Sensing A Neighbor UsingCoupling,” by Yan Li and Yupin Fong, Attorney Docket No. SAND-01083US9,filed on the same day as the present application;

“System That Compensates For Coupling Based On Sensing A Neighbor UsingCoupling,” by Yan Li and Yupin Fong, Attorney Docket No. SAND-01083US10,filed on the same day as the present application;

U.S. patent application Ser. No. 11/458,997, “Compensating For CouplingBased On Sensing A Neighbor Using Coupling,” filed Jul. 20, 2006, by YanLi and Yupin Fong, Attorney Docket No. SAND-01083US0;

U.S. patent application Ser. No. 11/459,001, “Compensating For CouplingDuring Programming,” filed Jul. 20, 2006, by Yan Li, Attorney Docket No.SAND-01083US2;

U.S. Pat. No. 7,400,535, “System That Compensates For Coupling DuringProgramming,” issued Jul. 15, 2008, by Yan Li, Attorney Docket No.SAND-01083US3;

U.S. patent application Ser. No. 12/124,434, “System That CompensatesFor Coupling During Programming,” filed May 21, 2008, by Yan Li,Attorney Docket No. SAND-01083US6;

U.S. patent application Ser. No. 12/124,440, “System That CompensatesFor Coupling During Programming,” filed May 21, 2008, 2008, by Yan Li,Attorney Docket No. SAND-01083US7;

U.S. patent application Ser. No. 11/458,996, “Method For ConfiguringCompensation,” filed Jul. 20, 2006, by Yan Li, Attorney Docket No.SAND-01083US4;

U.S. patent application Ser. No. 11/458,995, “System For ConfiguringCompensation,” filed Jul. 20, 2006, by Yan Li, Attorney Docket No.SAND-01083US5.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to technology for non-volatile memory.

2. Description of the Related Art

Semiconductor memory has become more popular for use in variouselectronic devices. For example, non-volatile semiconductor memory isused in cellular telephones, digital cameras, personal digitalassistants, mobile computing devices, non-mobile computing devices andother devices. Electrical Erasable Programmable Read Only Memory(EEPROM) and flash memory are among the most popular non-volatilesemiconductor memories.

Both EEPROM and flash memory utilize a floating gate that is positionedabove and insulated from a channel region in a semiconductor substrate.The floating gate is positioned between the source and drain regions. Acontrol gate is provided over and insulated from the floating gate. Thethreshold voltage of the transistor is controlled by the amount ofcharge that is retained on the floating gate. That is, the minimumamount of voltage that must be applied to the control gate before thetransistor is turned on to permit conduction between its source anddrain is controlled by the level of charge on the floating gate.

When programming an EEPROM or flash memory device, such as a NAND flashmemory device, typically a program voltage is applied to the controlgate and the bit line is grounded. Electrons from the channel areinjected into the floating gate. When electrons accumulate in thefloating gate, the floating gate becomes negatively charged and thethreshold voltage of the memory cell is raised so that the memory cellis in a programmed state. More information about programming can befound in U.S. Pat. No. 6,859,397, titled “Source Side Self-BoostingTechnique For Non-Volatile Memory,” and U.S. Pat. No. 6,917,545, titled“Detecting Over Programmed Memory,” both of which are incorporatedherein by reference in their entirety.

Some EEPROM and flash memory devices have a floating gate that is usedto store two ranges of charges and, therefore, the memory cell can beprogrammed/erased between two states (an erased state and a programmedstate). Such a flash memory device is sometimes referred to as a binaryflash memory device.

A multi-state flash memory device is implemented by identifying multipledistinct allowed/valid programmed threshold voltage ranges separated byforbidden ranges. Each distinct threshold voltage range corresponds to apredetermined value for the set of data bits encoded in the memorydevice.

Shifts in the apparent charge stored on a floating gate can occurbecause of the coupling of an electric field based on the charge storedin adjacent floating gates. This floating gate to floating gate couplingphenomena is described in U.S. Pat. No. 5,867,429, which is incorporatedherein by reference in its entirety. One example of an adjacent floatinggate to a target floating gate includes a floating gate that isconnected to the same word line and connect to an adjacent bit line.

The effect of the floating gate to floating gate coupling is of greaterconcern for multi-state devices because in multi-state devices theallowed threshold voltage ranges and the forbidden ranges are narrowerthan in binary devices. Therefore, the floating gate to floating gatecoupling can result in memory cells being shifted from an allowedthreshold voltage range to a forbidden range.

The floating gate to floating gate coupling can occur between sets ofadjacent memory cells that have been programmed at different times. Forexample, a first memory cell is programmed to add a level of charge toits floating gate that corresponds to one set of data. Subsequently, oneor more adjacent memory cells are programmed to add a level of charge totheir floating gates that correspond to a second set of data. After theone or more of the adjacent memory cells are programmed, the chargelevel read from the first memory cell appears to be different thanprogrammed because of the effect of the charge on the adjacent memorycells being coupled to the first memory cell. The coupling from adjacentmemory cells can shift the apparent charge level being read a sufficientamount to lead to an erroneous reading of the data stored.

The floating gate to floating gate coupling can also occur between setsof adjacent memory cells that have been programmed concurrently. Forexample, two adjacent multi-state memory cells may be programmed todifferent target levels such that a first memory cell is programmed to astate corresponding to a lower threshold voltage and a second memorycell is programmed to a state corresponding to a higher thresholdvoltage. The memory cell being programmed to the state corresponding tothe lower threshold voltage is likely to reach that state and be lockedout from further programming before the second memory cell reaches thestate corresponding to the higher threshold voltage. After the secondmemory cell reaches the state corresponding to the higher thresholdvoltage, it will couple to the first memory cell and cause the firstmemory cell to have a higher apparent threshold voltage than programmed.

As memory cells continue to shrink in size, the natural programming anderase distributions of threshold voltages are expected to increase dueto short channel effects, greater oxide thickness/coupling ratiovariations and more channel dopant fluctuations, thereby reducing theavailable separation between adjacent states This effect is much moresignificant for multi-state memories than memories using only two states(binary memories). Furthermore, the reduction of the space between wordlines and of the space between bit lines will also increase the couplingbetween adjacent floating gates.

Thus, there is a need to reduce the effects of coupling between adjacentfloating gates.

SUMMARY OF THE INVENTION

Technology is described herein for reducing the effect of couplingbetween adjacent floating gates. One embodiment includes reducing theeffect of coupling between adjacent floating gates on different bitlines; however, in other embodiments the technology described herein canbe used for coupling between other sets of floating gates (or other setsof memory cells that do not utilize floating gates).

One embodiment includes determining whether a set of non-volatilestorage elements are in a first condition and, in response, charging afirst set of control lines for non-volatile storage elements determinedto be in the first condition. A determination is made as to whichnon-volatile storage elements of the set have a neighbor non-volatilestorage element in the first condition by sensing a second set ofcontrol lines for non-volatile storage elements that are not in thefirst condition to determine whether first set of control lines havesufficiently coupled to the second set of control lines in response tothe charging. Non-volatile storage elements associated with controllines of the second set that have been sufficiently coupled to controllines of the first set have at least one neighbor in the firstcondition. In one example, the first condition corresponds to a flashmemory cell being in a most programmed state of a multi-statearchitecture. However, other programmed states or conditions can also beused.

Another embodiment includes applying a charge to a first set of controllines associated with a first set of non-volatile storage elements thatare in a first programmed condition and identifying a second set ofnon-volatile storage elements that have a neighbor non-volatile storageelement in the first programmed condition by detecting the charge in asecond set of control lines that are next to the first set of controllines. The second set of control lines are associated with the secondset of non-volatile storage elements.

Another embodiment includes sensing a condition of a neighbornon-volatile storage element based on capacitive coupling between aparticular control line for a particular non-volatile storage elementand a neighbor control line for the neighbor non-volatile storageelement. Information is read from the particular non-volatile storageelement while compensating for coupling between the neighbornon-volatile storage element and the particular non-volatile storageelement if the sensed condition is a first condition.

Another embodiment includes determining whether a set of non-volatilestorage elements are in a first condition and charging a first set ofcontrol lines for non-volatile storage elements determined to not be inthe first condition. A second set of control lines for non-volatilestorage elements determined to be in the first condition are charged.The charging of the second set of control lines is commencedsubsequently to commencing the charging of the first set of controllines so that at least a subset of the second set of control linescouple to at least a subset of the first set of control lines inresponse to the charging of the second set of control lines. Informationabout the non-volatile storage elements associated with the first set ofcontrol lines is sensed in response to charging of the first set ofcontrol lines and the charging of the second set of control lines.

The various methods described herein can be performed by variousdevices. One example of a suitable apparatus includes non-volatilestorage elements, control lines in communication with the non-volatilestorage elements, and a managing circuit in communication with thenon-volatile storage elements and the control lines. The managingcircuit performs the methods described herein. In various embodiments,the managing circuit includes any one or a combination of controlcircuitry, a power control circuit, decoders, a state machine, acontroller, and sense blocks. Other circuitry can also be included.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a NAND string.

FIG. 2 is an equivalent circuit diagram of the NAND string.

FIG. 3 is a cross-sectional view of the NAND string.

FIG. 4 is a block diagram of a portion of an array of NAND flash memorycells.

FIG. 5 is a block diagram of a non-volatile memory system.

FIG. 6 is a block diagram of a non-volatile memory system.

FIG. 7 is a block diagram of a memory array.

FIG. 8 is a block diagram depicting one embodiment of the sense block.

FIG. 9 is a schematic of one embodiment of a sense module.

FIG. 10 is a timing diagram for one embodiment of a sense module.

FIG. 11 is a flow chart describing one embodiment of a process forprogramming non-volatile memory.

FIG. 12 is an example wave form applied to the control gates ofnon-volatile memory cells.

FIG. 13 depicts an example set of threshold voltage distributions.

FIG. 14 depicts an example set of threshold voltage distributions.

FIG. 15 is a flow chart describing one embodiment of process that isperformed when reading data.

FIG. 16 is a flow chart describing one embodiment of process that isperformed when reading data.

FIG. 17 is a flow chart describing one embodiment of a process forsensing data from a neighboring bit line.

FIG. 18 is a timing diagram describing one embodiment of a process forsensing data from a neighboring bit line.

FIG. 19 is a flow chart describing one embodiment of a read process.

FIG. 20 depicts a distribution of threshold voltages for a programmedstate.

FIG. 21 is a graph that describes one embodiment of a programmingprocess.

FIG. 22 is a graph that describes one embodiment of a programmingprocess.

FIG. 23 is a flow chart describing one embodiment of a programmingprocess.

FIG. 24 is a flow chart describing one embodiment of a process forverifying.

FIG. 25 is a flow chart describing one embodiment of a process forreading data.

FIG. 26 is a timing diagram describing one embodiment for reading data.

FIG. 27 is a flow chart describing one embodiment of a processconfiguring and using a memory system.

FIG. 28 is a flow chart describing one embodiment of a process forconfiguring a memory system.

FIG. 29 is a flow chart describing one embodiment of a process forconfiguring a memory system.

FIG. 30 is a flow chart describing one embodiment of a process forconfiguring a memory system.

FIG. 31 is a flow chart describing one embodiment of a process forconfiguring a memory system.

DETAILED DESCRIPTION

One example of a memory system suitable for implementing the presentinvention uses the NAND flash memory structure, which includes arrangingmultiple transistors in series between two select gates. The transistorsin series and the select gates are referred to as a NAND string. FIG. 1is a top view showing one NAND string. FIG. 2 is an equivalent circuitthereof. The NAND string depicted in FIGS. 1 and 2 includes fourtransistors, 100, 102, 104 and 106, in series and sandwiched between afirst select gate 120 and a second select gate 122. Select gate 120gates the NAND string connection to bit line 126. Select gate 122 gatesthe NAND string connection to source line 128. Select gate 120 iscontrolled by applying the appropriate voltages to control gate 120CG.Select gate 122 is controlled by applying the appropriate voltages tocontrol gate 122CG. Each of the transistors 100, 102, 104 and 106 has acontrol gate and a floating gate. Transistor 100 has control gate 100CGand floating gate 100FG. Transistor 102 includes control gate 102CG andfloating gate 102FG. Transistor 104 includes control gate 104CG andfloating gate 104FG. Transistor 106 includes a control gate 106CG andfloating gate 106FG. Control gate 100CG is connected to (or is) wordline WL3, control gate 102CG is connected to word line WL2, control gate104CG is connected to word line WL1, and control gate 106CG is connectedto word line WL0. In one embodiment, transistors 100, 102, 104 and 106are each memory cells. In other embodiments, the memory cells mayinclude multiple transistors or may be different than that depicted inFIGS. 1 and 2. Select gate 120 is connected to select line SGD. Selectgate 122 is connected to select line SGS.

FIG. 3 provides a cross-sectional view of the NAND string describedabove. As depicted in FIG. 3, the transistors of the NAND string areformed in p-well region 140. Each transistor includes a stacked gatestructure that consists of a control gate (100CG, 102CG, 104CG and106CG) and a floating gate (100FG, 102FG, 104FG and 106FG). The controlgates and the floating gates are typically formed by depositingpolysilicon layers. The floating gates are formed on the surface of thep-well on top of an oxide or other dielectric film. The control gate isabove the floating gate, with an inter-polysilicon dielectric layerseparating the control gate and floating gate. The control gates of thememory cells (100, 102, 104 and 106) form the word lines. N+ dopeddiffusion regions 130, 132, 134, 136 and 138 are shared betweenneighboring cells, through which the cells are connected to one anotherin series to form a NAND string. These N+ doped regions form the sourceand drain of each of the cells. For example, N+ doped region 130 servesas the drain of transistor 122 and the source for transistor 106, N+doped region 132 serves as the drain for transistor 106 and the sourcefor transistor 104, N+ doped region 134 serves as the drain fortransistor 104 and the source for transistor 102, N+ doped region 136serves as the drain for transistor 102 and the source for transistor100, and N+ doped region 138 serves as the drain for transistor 100 andthe source for transistor 120. N+ doped region 126 connects to the bitline for the NAND string, while N+ doped region 128 connects to a commonsource line for multiple NAND strings.

Note that although FIGS. 1-3 show four memory cells in the NAND string,the use of four transistors is provided only as an example. A NANDstring used with the technology described herein can have less than fourmemory cells or more than four memory cells. For example, some NANDstrings will include 8 memory cells, 16 memory cells, 32 memory cells,64 memory cells, etc. The discussion herein is not limited to anyparticular number of memory cells in a NAND string.

Each memory cell can store data represented in analog or digital form.When storing one bit of digital data, the range of possible thresholdvoltages of the memory cell is divided into two ranges, which areassigned logical data “1” and “0.” In one example of a NAND-type flashmemory, the voltage threshold is negative after the memory cell iserased, and defined as logic “1.” The threshold voltage is positiveafter a program operation, and defined as logic “0.” When the thresholdvoltage is negative and a read is attempted by applying 0 volts to thecontrol gate, the memory cell will turn on to indicate logic one isbeing stored. When the threshold voltage is positive and a readoperation is attempted by applying 0 volts to the control gate, thememory cell will not turn on, which indicates that logic zero is stored.A memory cell storing one bit of digital data is referred to as a binarymemory cell.

A memory cell can also store multiple bits of digital data. Such amemory cell is referred to as a multi-state memory cell. The thresholdvoltage window for a multi-state memory cell is divided into the numberof states. For example, if four states are used, there will be fourthreshold voltage ranges assigned to the data values “11,” “10,” “01,”and “00.” In one example of a NAND-type memory, the threshold voltageafter an erase operation is negative and defined as “11.” Positivethreshold voltages are used for the states of “10,” “01,” and “00.”

Relevant examples of NAND-type flash memories and their operation areprovided in the following U.S. patents/patent applications, all of whichare incorporated herein by reference in their entirety: U.S. Pat. No.5,570,315; U.S. Pat. No. 5,774,397; U.S. Pat. No. 6,046,935; U.S. Pat.No. 5,386,422; U.S. Pat. No. 6,456,528; and U.S. patent application Ser.No. 09/893,277 (Publication No. US2003/0002348). Other types ofnon-volatile memory in addition to NAND flash memory can also be usedwith the present invention.

Another type of memory cell useful in flash EEPROM systems utilizes anon-conductive dielectric material in place of a conductive floatinggate to store charge in a non-volatile manner. Such a cell is describedin an article by Chan et al., “A True Single-TransistorOxide-Nitride-Oxide EEPROM Device,” IEEE Electron Device Letters, Vol.EDL-8, No. 3, March 1987, pp. 93-95. A triple layer dielectric formed ofsilicon oxide, silicon nitride and silicon oxide (“ONO”) is sandwichedbetween a conductive control gate and a surface of a semi-conductivesubstrate above the memory cell channel. The cell is programmed byinjecting electrons from the cell channel into the nitride, where theyare trapped and stored in a limited region. This stored charge thenchanges the threshold voltage of a portion of the channel of the cell ina manner that is detectable. The cell is erased by injecting hot holesinto the nitride. See also Nozaki et al., “A 1-Mb EEPROM with MONOSMemory Cell for Semiconductor Disk Application,” IEEE Journal ofSolid-State Circuits, Vol. 26, No. 4, April 1991, pp. 497-501, whichdescribes a similar cell in a split-gate configuration where a dopedpolysilicon gate extends over a portion of the memory cell channel toform a separate select transistor. The foregoing two articles areincorporated herein by reference in their entirety. The programmingtechniques mentioned in section 1.2 of “Nonvolatile Semiconductor MemoryTechnology,” edited by William D. Brown and Joe E. Brewer, IEEE Press,1998, incorporated herein by reference, are also described in thatsection to be applicable to dielectric charge-trapping devices. Thememory cells described in this paragraph can also be used with thepresent invention. Thus, the technology described herein also applies tocoupling between dielectric regions of different memory cells.

Another approach to storing two bits in each cell has been described byEitan et al., “NROM: A Novel Localized Trapping, 2-Bit NonvolatileMemory Cell,” IEEE Electron Device Letters, vol. 21, no. 11, November2000, pp. 543-545. An ONO dielectric layer extends across the channelbetween source and drain diffusions. The charge for one data bit islocalized in the dielectric layer adjacent to the drain, and the chargefor the other data bit localized in the dielectric layer adjacent to thesource. Multi-state data storage is obtained by separately readingbinary states of the spatially separated charge storage regions withinthe dielectric. The memory cells described in this paragraph can also beused with the present invention.

FIG. 4 illustrates an example of an array of NAND cells, such as thoseshown in FIGS. 1-3. Along each column, a bit line 206 is coupled to thedrain terminal 126 of the drain select gate for the NAND string 150.Along each row of NAND strings, a source line 204 may connect all thesource terminals 128 of the source select gates of the NAND strings. Anexample of a NAND architecture array and its operation as part of amemory system is found in U.S. Pat. Nos. 5,570,315; 5,774,397; and6,046,935.

The array of memory cells is divided into a large number of blocks ofmemory cells. As is common for flash EEPROM systems, the block is theunit of erase. That is, each block contains the minimum number of memorycells that are erased together. Each block is typically divided into anumber of pages. A page is a unit of programming. In one embodiment, theindividual pages may be divided into segments and the segments maycontain the fewest number of cells that are written at one time as abasic programming operation. One or more pages of data are typicallystored in one row of memory cells. A page can store one or more sectors.A sector includes user data and overhead data. Overhead data typicallyincludes an Error Correction Code (ECC) that has been calculated fromthe user data of the sector. A portion of the controller (describedbelow) calculates the ECC when data is being programmed into the array,and also checks it when data is being read from the array.Alternatively, the ECCs and/or other overhead data are stored indifferent pages, or even different blocks, than the user data to whichthey pertain. A sector of user data is typically 512 bytes,corresponding to the size of a sector in magnetic disk drives. Overheaddata is typically an additional 16-20 bytes. A large number of pagesform a block, anywhere from 8 pages, for example, up to 32, 64, 128 ormore pages.

FIG. 5 illustrates a memory device 296 having read/write circuits forreading and programming a page of memory cells in parallel, according toone embodiment of the present invention. Memory device 296 may includeone or more memory die 298. Memory die 298 includes a two-dimensionalarray of memory cells 300, control circuitry 310, and read/writecircuits 365. In some embodiments, the array of memory cells can bethree dimensional. The memory cell is controlled and accessed by variouscontrol lines, such as bit lines, word lines, source lines, and otherlines used to control the memory array. For example, the memory array300 is addressable by word lines via a row decoder 330 and by bit linesvia a column decoder 360. The read/write circuits 365 include multiplesense blocks 400 and allow a page of memory cells to be read orprogrammed in parallel. Typically a controller 350 is included in thesame memory device 296 (e.g., a removable storage card) as the one ormore memory die 298. Commands and Data are transferred between the hostand controller 350 via lines 320 and between the controller and the oneor more memory die 298 via lines 318.

The control circuitry 310 cooperates with the read/write circuits 365 toperform memory operations on the memory array 300. The control circuitry310 includes a state machine 312, an on-chip address decoder 314 and apower control module 316. The state machine 312 provides chip-levelcontrol of memory operations. The on-chip address decoder 314 providesan address interface between that used by the host or a memorycontroller to the hardware address used by the decoders 330 and 360. Thepower control module 316 controls the power and voltages supplied to theword lines and bit lines during memory operations.

In some implementations, some of the components of FIG. 5 can becombined. In various designs, one or more of the components of FIG. 5(alone or in combination), other than memory cell array 300, can bethought of as a managing circuit. For example, a managing circuits mayinclude any one of or a combination of control circuitry 310, statemachine 312, decoders 314/360, power control 316, sense blocks 400,read/write circuits 365, controller 350, etc.

FIG. 6 illustrates another arrangement of the memory device 296 shown inFIG. 5. Access to the memory array 300 by the various peripheralcircuits is implemented in a symmetric fashion, on opposite sides of thearray, so that the densities of access lines and circuitry on each sideare reduced by half. Thus, the row decoder is split into row decoders330A and 330B and the column decoder into column decoders 360A and 360B.Similarly, the read/write circuits are split into read/write circuits365A connecting to bit lines from the bottom and read/write circuits365B connecting to bit lines from the top of the array 300. In this way,the density of the read/write modules is essentially reduced by onehalf. The device of FIG. 6 can also include a controller, as describedabove for the device of FIG. 5.

With reference to FIG. 7 depicts an exemplary structure of memory cellarray 300 is described. As one example, a NAND flash EEPROM is describedthat is partitioned into 1,024 blocks. The data stored in each block canbe simultaneously erased. In one embodiment, the block is the minimumunit of memory cells that are simultaneously erased. In each block, inthis example, there are 8,512 columns corresponding to bit lines BL0,BL1, . . . BL8511. In one embodiment, all the bit lines of a block canbe simultaneously selected during read and program operations. Memorycells along a common word line and connected to any bit line can beprogrammed at the same time.

In another embodiment, the bit lines are divided into even bit lines andodd bit lines. In an odd/even bit line architecture, memory cells alonga common word line and connected to the odd bit lines are programmed atone time, while memory cells along a common word line and connected toeven bit lines are programmed at another time.

FIG. 7 shows four memory cells connected in series to form a NANDstring. Although four cells are shown to be included in each NANDstring, more or less than four can be used (e.g., 16, 32, or anothernumber). One terminal of the NAND string is connected to a correspondingbit line via a drain select gate (connected to select gate drain lineSGD), and another terminal is connected to c-source via a source selectgate (connected to select gate source line SGS).

FIG. 8 is a block diagram of an individual sense block 400 partitionedinto a core portion, referred to as a sense module 380, and a commonportion 390. In one embodiment, there will be a separate sense module380 for each bit line and one common portion 390 for a set of multiplesense modules 380. In one example, a sense block will include one commonportion 390 and eight sense modules 380. Each of the sense modules in agroup will communicate with the associated common portion via a data bus372. For further details, refer to U.S. patent application Ser. No.11/026,536 “Non-Volatile Memory & Method with Shared Processing for anAggregate of Sense Amplifiers” filed on Dec. 29, 2004, which isincorporated herein by reference in its entirety.

Sense module 380 comprises sense circuitry 370 that determines whether aconduction current in a connected bit line is above or below apredetermined threshold level. Sense module 380 also includes a bit linelatch 382 that is used to set a voltage condition on the connected bitline. For example, a predetermined state latched in bit line latch 382will result in the connected bit line being pulled to a statedesignating program inhibit (e.g., Vdd).

Common portion 390 comprises a processor 392, a set of data latches 394and an I/O Interface 396 coupled between the set of data latches 394 anddata bus 320. Processor 392 performs computations. For example, one ofits functions is to determine the data stored in the sensed memory celland store the determined data in the set of data latches. The set ofdata latches 394 is used to store data bits determined by processor 392during a read operation. It is also used to store data bits importedfrom the data bus 320 during a program operation. The imported data bitsrepresent write data meant to be programmed into the memory. I/Ointerface 396 provides an interface between data latches 394 and thedata bus 320.

During read or sensing, the operation of the system is under the controlof state machine 312 that controls the supply of different control gatevoltages to the addressed cell. As it steps through the variouspredefined control gate voltages corresponding to the various memorystates supported by the memory, the sense module 380 may trip at one ofthese voltages and an output will be provided from sense module 380 toprocessor 392 via bus 372. At that point, processor 392 determines theresultant memory state by consideration of the tripping event(s) of thesense module and the information about the applied control gate voltagefrom the state machine via input lines 393. It then computes a binaryencoding for the memory state and stores the resultant data bits intodata latches 394. In another embodiment of the core portion, bit linelatch 382 serves double duty, both as a latch for latching the output ofthe sense module 380 and also as a bit line latch as described above.

It is anticipated that some implementations will include multipleprocessors 392. In one embodiment, each processor 392 will include anoutput line (not depicted in FIG. 9) such that each of the output linesis wired-OR'd together. In some embodiments, the output lines areinverted prior to being connected to the wired-OR line. Thisconfiguration enables a quick determination during the programverification process of when the programming process has completedbecause the state machine receiving the wired-OR can determine when allbits being programmed have reached the desired level. For example, wheneach bit has reached its desired level, a logic zero for that bit willbe sent to the wired-OR line (or a data one is inverted). When all bitsoutput a data 0 (or a data one inverted), then the state machine knowsto terminate the programming process. Because each processorcommunicates with eight sense modules, the state machine needs to readthe wired-OR line eight times, or logic is added to processor 392 toaccumulate the results of the associated bit lines such that the statemachine need only read the wired-OR line one time. Similarly, bychoosing the logic levels correctly, the global state machine can detectwhen the first bit changes its state and change the algorithmsaccordingly.

During program or verify, the data to be programmed is stored in the setof data latches 394 from the data bus 320. The program operation, underthe control of the state machine, comprises a series of programmingvoltage pulses applied to the control gates of the addressed memorycells. Each programming pulse is followed by a verify operation todetermine if the memory cell has been programmed to the desired state.Processor 392 monitors the verified memory state relative to the desiredmemory state. When the two are in agreement, the processor 222 sets thebit line latch 214 so as to cause the bit line to be pulled to a statedesignating program inhibit. This inhibits the cell coupled to the bitline from further programming even if programming pulses appear on itscontrol gate. In other embodiments the processor initially loads the bitline latch 382 and the sense circuitry sets it to an inhibit valueduring the verify process.

Data latch stack 394 contains a stack of data latches corresponding tothe sense module. In one embodiment, there are three data latches persense module 380. In some implementations (but not required), the datalatches are implemented as a shift register so that the parallel datastored therein is converted to serial data for data bus 320, and viceversa. In the preferred embodiment, all the data latches correspondingto the read/write block of m memory cells can be linked together to forma block shift register so that a block of data can be input or output byserial transfer. In particular, the bank of r read/write modules isadapted so that each of its set of data latches will shift data in to orout of the data bus in sequence as if they are part of a shift registerfor the entire read/write block.

Additional information about the structure and/or operations of variousembodiments of non-volatile storage devices can be found in (1) UnitedStates Patent Application Pub. No. 2004/0057287, “Non-Volatile MemoryAnd Method With Reduced Source Line Bias Errors,” published on Mar. 25,2004; (2) United States Patent Application Pub No. 2004/0109357,“Non-Volatile Memory And Method with Improved Sensing,” published onJun. 10, 2004; (3) U.S. patent application Ser. No. 11/015,199 titled“Improved Memory Sensing Circuit And Method For Low Voltage Operation,”Inventor Raul-Adrian Cernea, filed on Dec. 16, 2004; (4) U.S. patentapplication Ser. No. 11/099,133, titled “Compensating for CouplingDuring Read Operations of Non-Volatile Memory,” Inventor Jian Chen,filed on Apr. 5, 2005; and (5) U.S. patent application Ser. No.11/321,953, titled “Reference Sense Amplifier For Non-Volatile Memory,Inventors Siu Lung Chan and Raul-Adrian Cernea, filed on Dec. 28, 2005.All five of the immediately above-listed patent documents areincorporated herein by reference in their entirety.

FIG. 9 illustrates an example of sense module 380; however, otherimplementations can also be used. Sense module 380 comprises bit lineisolation transistor 512, bit line pull down circuit (transistors 522and 550), bit line voltage clamp transistor 612, readout bus transfergate 530, sense amplifier 600 and bit line latch 382. One side of bitline isolation transistor 512 is connected to the bit line BL andcapacitor 510. The other side of bit line isolation transistor 512 isconnected to bit line voltage clamp transistor 612 and bit line pulldown transistor 522. The gate of bit line isolation transistor 512receives a signal labeled as BLS. The gate of bit line voltage clamptransistor 512 receives a signal labeled as BLC. Bit line voltage clamptransistor 512 is connected to readout bus transfer gate 530 at nodeSEN2. Readout bus transfer gate 530 is connected to readout bus 532. Bitline voltage clamp transistor 512 connects to sense amplifier 600 atnode SEN2. In the embodiment of FIG. 9, sense amplifier 600 includestransistors 613, 634, 641, 642, 643,654, 654 and 658, as well ascapacitor Csa, Bit line latch 382 includes transistors 661, 662, 663,664, 666 and 668.

In general, memory cells along a word line are operated on in parallel.Therefore a corresponding number of sense modules are in operation inparallel. In one embodiment, a controller provides control and timingsignals to the sense modules operating in parallel. In some embodiments,data along a word line is divided into multiple pages, and the data isread or programmed a page at a time, or multiple pages at a time.

Sense module 380 is connectable to the bit line (e.g., bit line BL) fora memory cell when the bit line isolation transistor 512 is enabled bysignal BLS. Sense module 380 senses the conduction current of the memorycell by means of sense amplifier 600 and latches the read result as adigital voltage level at a sense node SEN2 and outputs it to readout bus532 via gate 530.

The sense amplifier 600 comprises a second voltage clamp (transistors612 and 634), a pre-charge circuit (transistors 541, 642 and 643), and adiscriminator or compare circuit (transistors 654, 656 and 658; andcapacitor Csa). In one embodiment, a reference voltage is applied to thecontrol gate of a memory cell being read. If the reference voltage isgreater than the threshold voltage of the memory cell, then the memorycell will turn on and conduct current between its source and drain. Ifthe reference voltage is not greater than the threshold voltage of thememory cell, then the memory cell will not turn on and will not conductcurrent between its source and drain. In many implementations, theon/off may be a continuous transition so that the memory cell willconduct different currents in response to different control gatevoltages. If the memory cell is on and conducting current, the conductedcurrent will cause the voltage on node SEN to decrease, effectivelycharging or increasing the voltage across capacitor Csa whose otherterminal is at Vdd. If the voltage on node SEN discharges to apredetermined level during a predetermined sensing period, then senseamplifier 600 reports that the memory cell turned on in response to thecontrol gate voltage.

One feature of the sense module 380 is the incorporation of a constantvoltage supply to the bit line during sensing. This is preferablyimplemented by the bit line voltage clamp transistor 612, which operateslike a diode clamp with transistor 612 in series with the bit line BL.Its gate is biased to a constant voltage BLC equal to the desired bitline voltage VBL above its threshold voltage VT. In this way, itisolates the bit line from the node SEN and sets a constant voltagelevel for the bit line, such as the desired VBL=0.5 to 0.7 volts duringprogram-verifying or reading. In general, the bit line voltage level isset to a level such that it is sufficiently low to avoid a longprecharge time, yet sufficiently high to avoid ground noise and otherfactors.

Sense amplifier 600 senses the conduction current through the sense nodeSEN and determines whether the conduction current is above or below apredetermined value. The sense amplifier outputs the sensed result in adigital form as the signal SEN2 to readout bus 532.

The digital control signal INV, which can essentially be an invertedstate of the signal at SEN2, is also output to control the pull downcircuit. When the sensed conduction current is higher than thepredetermined value, INV will be HIGH and SEN2 will be LOW. This resultis reinforced by the pull down circuit. The pull down circuit includesan n-transistor 522 controlled by the control signal INV and anothern-transistor 550 controlled by the control signal GRS. The GRS signalwhen LOW allows the bit line BL to be floated regardless of the state ofthe INV signal. During programming, the GRS signal goes HIGH to allowthe bit line BL to be pulled to ground and controlled by INV. When thebit line BL is required to be floated, the GRS signal goes LOW. Notethat other designs of sense modules, sense amplifiers and latches canalso be used.

Read/write circuits 365 operate on a page of memory cellssimultaneously. Each sense module 380 in the read/write circuits 365 iscoupled to a corresponding cell via a bit line. The conduction currentflows from the sense module through the bit line into the drain of thememory cell and out from the source before going through a source lineto ground. In an integrated circuit chip, the sources of the cells in amemory array are all tied together as multiple branches of the sourceline connected to some external ground pad (e.g., Vss pad) of the memorychip. Even when metal strapping is used to reduce the resistance of thesource line, a finite resistance, R, remains between the sourceelectrode of a memory cell and the ground pad. Typically, the groundloop resistance R is around 50 ohm.

For the entire page of memory being sensed in parallel, the totalcurrent flowing through the source line is the sum of all the conductioncurrents. Generally, each memory cell has a conduction current dependenton the amount of charge programmed into its charge storage element. Fora given control gate voltage of the memory cell, a small charge willyield a comparatively higher conduction current. When a finiteresistance exists between the source electrode of a memory cell and theground pad, the voltage drop across the resistance is given byVdrop=iTOT R.

For example, if 4,256 bit lines discharge at the same time, each with acurrent of 1 μA, then the source line voltage drop will be equal to4,000 lines×1 μA/line×50 ohms ˜0.2 volts. This source line bias willcontribute to a sensing error of 0.2 volts when threshold voltages ofthe memory cells are sensed.

In one set of embodiments, a method for reducing source line bias isaccomplished by read/write circuits with features and techniques formulti-pass sensing. Each pass helps to identify and shut down the memorycells with conduction current higher than a given demarcation currentvalue. Typically, with each pass, the given demarcation current valueprogressively converges to the breakpoint current value for aconventional single-pass sensing. In this way, sensing in subsequentpasses will be less affected by source line bias since the highercurrent cells have been shut down.

For example, multi-pass sensing can be implemented in two passes (j=0 to1). After the first pass, those memory cells with conduction currentshigher than the breakpoint are identified and removed by turning offtheir conduction current. A preferred way to turn off their conductioncurrents is to set their drain voltages on their bit lines to ground. Ina second pass, error from source line bias is reduced. More than twopasses are also contemplated. In embodiments where source line bias willnot cause errors, one pass can be used for sensing.

FIGS. 10(A)-10(K) depicts timing diagrams that explain one embodiment ofsense module 380 during read/verify operations.

Phase (0): Setup

The sense module 380 (see FIG. 9) is connected to the corresponding bitline via an enabling signal BLS (FIG. 10(A)). The Voltage clamp isenabled with BLC (FIG. 10(B)). Pre-charge transistor 642 is enabled as alimited-current source with a control signal FLT (FIG. 10(C)).

Phase (1): Controlled Pre-Charge

Sense amplifier 600 is initialized by a reset signal RST (FIG. 10(D))which will pull the signal INV (FIG. 10(H)) to ground via transistor658. Thus, on reset, INV is set to LOW. At the same time, p-transistor663 pulls a complementary signal LAT to V_(dd) or HIGH (FIG. 10(H)).That is, LAT is the complement of INV. Isolation transistor 634 iscontrolled by the signal LAT. Thus, after reset, isolation transistor634 is enabled to connect sense node SEN2 to the sense amplifier'sinternal sense node SEN.

Pre-charge transistor 642 pre-charges the bit line BL through theinternal sense node SEN and the sense node SEN2 for a predeterminedperiod of time. This will bring the bit line to an optimal voltage forsensing the conduction therein. Pre-charge transistor 642 is controlledby the control signal FLT (“FLOAT”). The bit line will be pulled uptowards the desired bit line voltage as set by the bit line voltageclamp 612. The rate of pull-up will depend on the conduction current inthe bit line. The smaller the conduction current, the faster thepull-up.

It has been described earlier that sensing errors due to the source linebias are minimized if those memory cells with conduction currents higherthan a predetermined value are turned off and their contributions to thesource line bias eliminated. Pre-charge transistor 642, therefore,serves two functions. One is to pre-charge the bit line to an optimalsensing voltage. The other is to help identify those memory cells withconduction currents higher than a predetermined value for D.C. (DirectCurrent) sensing so that they may be eliminated from contributing tosource line bias.

The D.C. sensing is accomplished by providing a pre-charge circuit thatbehaves like a current source for supplying a predetermined current tothe bit line. The signal FLT that controls the p-transistor 642 is suchthat it “programs” a predetermined current to flow. As an example, theFLT signal may be generated from a current mirror with a referencecurrent set to 500 nA. When the p-transistor 642 forms the mirrored legof the current mirror, it will also have the same 500 nA throwing in it.

FIGS. 10(I1)-10(I4) illustrate the voltages on four example bit linesconnected respectively to memory cells with conduction currents of 700nA, 400 nA, 220 nA and 40 nA. When a pre-charge circuit (which includestransistor 642) is a current source with a limit of 500 nA, for example,a memory cell having a conduction current exceeding 500 nA will have thecharges on the bit line drained faster than it can accumulate.Consequently, for the bit line with conduction current 700 nA, itsvoltage or the signal at the internal sense node SEN will remain closeto 0V (such as 0.1 volt; see FIG. 10(I1)). On the other hand, if thememory cell's conduction current is below 500 nA, the pre-charge circuit(which includes transistor 642) will begin to charge up the bit line andits voltage will begin to rise towards the clamped bit line voltage(e.g., 0.5V set by the voltage clamp 612) (FIGS. 10(I2)-10(I4)).Correspondingly, the internal sense node SEN will either remain close to0V or be pulled up to Vdd (FIG. 10(G)). Generally, the smaller theconduction current, the faster the bit line voltage will charge up tothe clamped bit line voltage. Thus, by examining the voltage on a bitline after the controlled precharge phase, it is possible to identify ifthe connected memory cell has a conduction current higher or lower thana predetermined level.

Phase (2): D.C. Latching & Removing High Current Cells from SubsequentStrobes

After the controlled pre-charge phase, an initial D.C. high-currentsensing phase begins where the node SEN is sensed by the discriminatorcircuit. The sensing identifies those memory cells with conductioncurrents higher than the predetermined level. The discriminator circuitincludes two p-transistors 654 and 656 in series, which serve as apull-up for a node registering the signal INV. The p-transistor 654 isenabled by a read strobe signal STB going LOW and the p-transistor 656is enabled by the signal at the internal sense node SEN going LOW. Highcurrent memory cells will have the signal SEN close to 0V or at leastunable for its bit lines to be pre-charged sufficiently high to turn offthe p-transistor 656. For example, if the weak pull up is limited to acurrent of 500 nA, it will fail to pull up a cell with conductioncurrent of 700 nA (FIG. 10(G1)). When STB strobes LOW to latch, INV ispulled up to V_(dd). This will set the latch circuit 660 with INV HIGHand LAT LOW (FIG. 10(H1)).

When INV is HIGH and LAT LOW, the isolation gate 630 is disabled and thesense node SEN2 is blocked from the internal sense node SEN. At the sametime, the bit line is pulled to ground by the pull down transistor 522(FIGS. 9 & 10(I1)). This will effectively turn off any conductioncurrent in the bit line, eliminating it from contributing to source linebias.

Thus, in one preferred implementation of the sense module 380, alimited-current source pre-charge circuit is employed. This provides anadditional or alternative way (D.C. sensing) to identify bit linescarrying high currents and to turn them off to minimize source line biaserror in subsequent sensing.

In another embodiment, the pre-charge circuit is not specificallyconfigured to help identify high current bit lines but is optimized topull up and pre-charge the bit line as fast as possible within theallowance of the maximum current available to the memory system.

Phase (3): Recovery/Pre-Charge

Prior to a sensing of the conduction current in a bit line that has notbeen previously pulled down, the pre-charge circuit is activated by thesignal FLT going LOW to pre-charge the internal sense node SEN2 toV_(dd) (FIG. 10(C) and FIGS. 10(I2)-10(I4)) and the bit line which mayhave been partially coupled down due to a decrease in the voltage onadjacent bit lines.

Phase (4): First A.C. Sensing

In one embodiment, an A.C. (Alternating Current or transient) sensing isperformed by determining the voltage drop at the floated internal sensenode SEN. This is accomplished by the discriminator circuit employingthe capacitor Csa coupled to the internal sense node SEN, andconsidering the rate the conduction current is charging it (reducing thevoltage on node SEN). In an integrated circuit environment, thecapacitor Csa is typically implemented with a transistor; however, otherimplementations are suitable. Capacitor Csa has a predeterminedcapacitance, e.g., 30 fF, which can be selected for optimal currentdetermination. The demarcation current value, typically in the range of100-1000 nA, can be set by appropriate adjustment of the chargingperiod.

The discriminator circuit senses the signal SEN in the internal sensenode SEN. Prior to each sensing, the signal at the internal sense nodeSEN is pulled up to V_(dd) by pre0charge transistor 642. This willinitially set the voltage across the capacitor Csa to be zero.

When the sense amplifier 600 is ready to sense, the precharge circuit isdisabled by FLT going HIGH (FIG. 10(C)). The first sensing period T1 isended by the assertion of the strobe signal STB. During the sensingperiod, a conduction current induced by a conducting memory cell willcharge the capacitor. The voltage at SEN will decrease from V_(dd) asthe capacitor Csa is charged through the draining action of theconduction current in the bit line. FIG. 10(G) (see curves G2-G4)illustrates the SEN node corresponding to the remaining three examplebit lines connected respectively to memory cells with conductioncurrents of 400 nA, 220 nA and 40 nA, the decrease being more rapid forthose with a higher conduction current.

Phase (5): First A.C. Latching and Removal of Higher Current Cells fromSubsequent Sensing

At the end of the first predetermined sensing period, the SEN node willhave decreased to some voltage depending on the conduction current inthe bit line (see curves G2-G4 of FIG. 10G). As an example, thedemarcation current in this first phase is set to be at 300 nA. Thecapacitor Csa, the sensing period T1 and the threshold voltage of thep-transistor 656 are such that the signal at SEN corresponding to aconduction current higher than the demarcation current (e.g., 300 nA)will drop sufficiently low to turn on the transistor 656. When latchingsignal STB strobes LOW, the output signal INV will be pulled HIGH, andwill be latched by the latch 382 (FIG. 10(E) and FIG. 10(H) (curve H2)).On the other hand, the signal SEN corresponding to a conduction currentbelow the demarcation current will produce a signal SEN unable to turnon the transistor 656. In this case, the latch 382 will remainunchanged, in which case LAT remains HIGH (FIGS. 10(H3) and 10(H4)).Thus, it can be seen that the discriminator circuit effectivelydetermines the magnitude of the conduction current in the bit linerelative to a reference current set by the sensing period.

Sense amplifier 600 also includes the second voltage clamp transistor612 whose purpose is to maintain the voltage of the drain of thetransistor 612 sufficiently high in order for the bit line voltage clamp610 to function properly. As described earlier, the bit line voltageclamp 610 clamps the bit line voltage to a predetermined value V_(BL),e.g., 0.5V. This will require the gate voltage BLC of the transistor 612to be set at V_(BL)+V_(T) (where V_(T) is the threshold voltage of thetransistor 612) and the drain connected to the sense node 501 to begreater than the source, i.e., the signal SEN2>V_(BL). In particular,given the configurations of the voltage clamps, SEN2 should be no higherthan the smaller of XX0−V_(T) or BLX−V_(T), and SEN should be no lower.During sensing, the isolation gate 630 is in a pass-through mode.However, during sensing the signal at the internal sense node SEN has avoltage that decreases from V_(dd). The second voltage clamp preventsSEN from dropping below XX0−V_(T) or BLX−V_(T), whichever is lower. Thisis accomplished by an n-transistor 612 controlled by a signal BLX, whereBLX is ≧V_(BL)+V_(T). Thus, through the actions of the voltage clamps,the bit line voltage V_(BL) is kept constant, e.g., ˜0.5V, duringsensing.

The output of the current determination is latched by the latch circuit382. The latch circuit is formed as a Set/Reset latch by the transistors661, 662, 663 and 664 together with the transistors 666 and 668. Thep-transistor 666 is controlled by the signal RST (RESET) and then-transistor 668 is controlled by the signal STB. A variation of theabove-described sense amplifier that is adapted for low voltageoperation is found in U.S. patent application Ser. No. 11/015,199 titled“Improved Memory Sensing Circuit And Method For Low Voltage Operation,”Inventor Raul-Adrian Cernea, filed on Dec. 16, 2004, incorporated hereinby reference in its entirety.

In general, there will be a page of memory cells being operated on by acorresponding number of multi-pass sense modules 380. For those memorycells having conduction current higher than the first demarcationcurrent level, their LAT signal will be latched LOW (INV latched HIGH).This in turn activates the bit line pull down circuit 520 to pull thecorresponding bit lines to ground, thereby turning off their currents.

Phase (6): Recovery/Pre-Charge

Prior to the next sensing of the conduction current in a bit line thathas not been previously pulled down, the pre-charge circuit is activatedby the signal FLT to pre-charge the internal sense node 631 to V_(dd)(FIG. 10(C) and FIGS. 10(I3)-10(I4)).

Phase (7): Second Sensing

When the sense amplifier 600 is ready to sense, the pre-charge circuitis disabled by FLT going HIGH (FIG. 10(C)). The second sensing period T2is set by the assertion of the strobe signal STB. During the sensingperiod, a conduction current, if any, will charge the capacitor. Thesignal at the node SEN will decrease from V_(dd) as capacitor Csa ischarging through the draining action of the conduction current in thebit line 36.

In accordance with the example before, the memory cells with conductioncurrents higher than 300 nA have already been identified and shut downin the earlier phases. FIG. 14(G) (curves G3 and G4) illustraterespectively the SEN signal corresponding to the two example bit linesconnected respectively to memory cells with conduction currents of 220nA and 40 nA.

Phase (8): Second Latching for Reading Out

At the end of the second predetermined sensing period T2, SEN will havedecreased to some voltage depending on the conduction current in the bitline (FIG. 10(G) (curves G3 and G4)). As an example, the demarcationcurrent in this second phase is set to be at 100 nA. In this case, thememory cell with the conduction current 220 nA will have its INV latchedHIGH (FIG. 10(H)) and its bit line subsequently pulled to ground (FIG.10(I3)). On the other hand, the memory cell with the conduction current40 nA will have no effect on the state of the latch, which was presetwith LAT HIGH.

Phase (9): Read Out to the Bus

Finally, in the read out phase, the control signal NCO at the transfergate 530 allows the latched signal SEN2 to be read out to the readoutbus 532 (FIGS. 10(J) and 10(K)).

As can be seen from FIGS. 10(I1)-10(I4), the bit line voltage remainsconstant during each sensing period. Thus, from the discussion earlier,capacitive bit-line to bit-line coupling is eliminated.

The sense module 380 described above is one embodiment where sensing isperformed with three passes, the first two passes being implemented toidentify and shut down higher current memory cells. With the highercurrent contributions to the source line bias eliminated, the final passis able to sense the cells with lower range conduction currents moreaccurately.

In other embodiments, sensing operations are implemented with differentcombination of D.C. and A.C. passes, some using only two or more A.C.passes, or only one pass. For the different passes, the demarcationcurrent value used may be the same each time or converge progressivelytowards the demarcation current used in the final pass. Additionally,the sensing embodiment described above is just one example of a suitablesense module. Other designs and technologies can also be used toimplement the invention described herein. No one particular sense moduleis required or suggested for the invention described herein.

FIG. 11 is a flow chart describing one embodiment of a method forprogramming non-volatile memory. In one implementation, memory cells areerased (in blocks or other units) prior to programming. Memory cells areerased in one embodiment by raising the p-well to an erase voltage(e.g., 20 volts) for a sufficient period of time and grounding the wordlines of a selected block while the source and bit lines are floating.Due to capacitive coupling, the unselected word lines, bit lines, selectlines, and c-source are also raised to a significant fraction of theerase voltage. A strong electric field is thus applied to the tunneloxide layers of selected memory cells and the data of the selectedmemory cells are erased as electrons of the floating gates are emittedto the substrate side, typically by Fowler-Nordheim tunneling. Aselectrons are transferred from the floating gate to the p-well region,the threshold voltage of a selected cell is lowered. Erasing can beperformed on the entire memory array, separate blocks, or another unitof cells.

In step 700 of FIG. 11, a “data load” command is issued by thecontroller and received by control circuitry 310. In step 702, addressdata designating the page address is input to decoder 314 from thecontroller or host. In step 704, a page of program data for theaddressed page is input to a data buffer for programming. That data islatched in the appropriate set of latches. In step 706, a “program”command is issued by the controller to state machine 312.

Triggered by the “program” command, the data latched in step 704 will beprogrammed into the selected memory cells controlled by state machine312 using the stepped pulses of FIG. 12 applied to the appropriate wordline. In step 708, the program voltage Vpgm is initialized to thestarting pulse (e.g., 12V or other value) and a program counter PCmaintained by state machine 312 is initialized at 0. In step 710, thefirst Vpgm pulse is applied to the selected word line. If logic “0” isstored in a particular data latch indicating that the correspondingmemory cell should be programmed, then the corresponding bit line isgrounded. On the other hand, if logic “1” is stored in the particularlatch indicating that the corresponding memory cell should remain in itscurrent data state, then the corresponding bit line is connected to Vddto inhibit programming.

In step 712, the states of the selected memory cells are verified todetermine if they have reached their target threshold voltage. If it isdetected that the threshold voltage of a selected cell has reached thetarget level, then the data stored in the corresponding data latch ischanged to a logic “1.” If it is detected that the threshold voltage hasnot reached the appropriate level, the data stored in the correspondingdata latch is not changed. In this manner, a bit line having a logic “1”stored in its corresponding data latch does not need to be programmed.When all of the data latches are storing logic “1,” the state machine(via the wired-OR type mechanism described above) knows that allselected cells have been programmed. In step 714, it is checked whetherall of the data latches are storing logic “1.” If so, the programmingprocess is complete and successful because all selected memory cellswere programmed and verified. A status of “PASS” is reported in step716.

If, in step 714, it is determined that not all of the data latches arestoring logic “1,” then the programming process continues. In step 718,the program counter PC is checked against a program limit value PCMAX.One example of a program limit value is 20; however, other numbers canalso be used. If the program counter PC is not less than 20, then theprogram process has failed and a status of “FAIL” is reported in step720. In some embodiments, after the maximum number of loops is reached,the system checks whether less than a predetermined amount of cells havenot finished programming. If less than that predetermined number has notfinished programming, the programming process is still considered pass.If the program counter PC is less than 20, then the Vpgm level isincreased by the step size and the program counter PC is incremented instep 722. After step 722, the process loops back to step 710 to applythe next Vpgm pulse.

FIG. 12 shows a series of program pulses that are applied to the wordline selected for programming. In between program pulses are a set ofverify pulses (not depicted). In some embodiments, there can be a verifypulse for each state that data is being programmed into. In otherembodiments, there can be more or less verify pulses.

In one embodiment, data is programmed to memory cells along a commonword line. Thus, prior to applying the program pulses of FIG. 12, one ofthe word lines is selected for programming. This word line will bereferred to as the selected word line. The remaining word lines of ablock are referred to as the unselected word lines.

At the end of a successful program (with verification) process, thethreshold voltages of the memory cells should be within one or moredistributions of threshold voltages for programmed memory cells orwithin a distribution of threshold voltages for erased memory cells, asappropriate. FIG. 13 illustrates example threshold voltage distributionsfor the memory cell array when each memory cell stores two bits of data.FIG. 13 shows a first threshold voltage distribution E for erased memorycells. Three threshold voltage distributions, A, B and C for programmedmemory cells, are also depicted. In one embodiment, the thresholdvoltages in the E distribution are negative and the threshold voltagesin the A, B and C distributions are positive.

Each distinct threshold voltage range of FIG. 13 corresponds topredetermined values for the set of data bits. The specific relationshipbetween the data programmed into the memory cell and the thresholdvoltage levels of the cell depends upon the data encoding scheme adoptedfor the cells. For example, U.S. Pat. No. 6,222,762 and U.S. PatentApplication Publication No. 2004/0255090, “Tracking Cells For A MemorySystem,” filed on Jun. 13, 2003, both of which are incorporated hereinby reference in their entirety, describe various data encoding schemesfor multi-state flash memory cells. In one embodiment, data values areassigned to the threshold voltage ranges using a Gray code assignment sothat if the threshold voltage of a floating gate erroneously shifts toits neighboring physical state, only one bit will be affected. Oneexample assigns “11” to threshold voltage range E (state E), “10” tothreshold voltage range A (state A), “00” to threshold voltage range B(state B) and “01” to threshold voltage range C (state C). However, inother embodiments, Gray code is not used. Although FIG. 13 shows fourstates, the present invention can also be used with other multi-statestructures including those that include more or less than four states.For example, some non-volatile storage elements can utilize eight (sevenprogrammed and one erased) or more states.

FIG. 13 also shows three read reference voltages, Vra, Vrb and Vrc, forreading data from memory cells. By testing whether the threshold voltageof a given memory cell is above or below Vra, Vrb and Vrc, the systemcan determine what state the memory cell is in.

FIG. 13 also shows three verify reference voltages, Vva, Vvb and Vvc.When programming memory cells to state A, the system will test whetherthose memory cells have a threshold voltage greater than or equal toVva. When programming memory cells to state B, the system will testwhether the memory cells have threshold voltages greater than or equalto Vvb. When programming memory cells to state C, the system willdetermine whether memory cells have their threshold voltage greater thanor equal to Vvc.

In one embodiment, known as full sequence programming, memory cells canbe programmed from the erase state E directly to any of the programmedstates A, B or C. For example, a population of memory cells to beprogrammed may first be erased so that all memory cells in thepopulation are in erased state E. While some memory cells are beingprogrammed from state E to state A, other memory cells are beingprogrammed from state E to state B and/or from state E to state C.

FIG. 14 illustrates an example of a two-pass technique of programming amulti-state memory cell that stores data for two different pages: alower page and an upper page. Four states are depicted: state E (11),state A (10), state B (00) and state C (01). For state E, both pagesstore a “1.” For state A, the lower page stores a “0” and the upper pagestores a “1.” For state B, both pages store “0.” For state C, the lowerpage stores “1” and the upper page stores “0.” Note that althoughspecific bit patterns have been assigned to each of the states,different bit patterns may also be assigned.

In a first programming pass, the cell's threshold voltage level is setaccording to the bit to be programmed into the lower logical page. Ifthat bit is a logic “1,” the threshold voltage is not changed since itis in the appropriate state as a result of having been earlier erased.However, if the bit to be programmed is a logic “0,” the threshold levelof the cell is increased to be state A, as shown by arrow 730.

In a second programming pass, the cell's threshold voltage level is setaccording to the bit being programmed into the upper logical page. Ifthe upper logical page bit is to store a logic “1,” then no programmingoccurs since the cell is in one of the states E or A, depending upon theprogramming of the lower page bit, both of which carry an upper page bitof “1.” If the upper page bit is to be a logic “0,” then the thresholdvoltage is shifted. If the first pass resulted in the cell remaining inthe erased state E, then in the second phase the cell is programmed sothat the threshold voltage is increased to be within state C, asdepicted by arrow 734. If the cell had been programmed into state A as aresult of the first programming pass, then the memory cell is furtherprogrammed in the second pass so that the threshold voltage is increasedto be within state B, as depicted by arrow 732. The result of the secondpass is to program the cell into the state designated to store a logic“0” for the upper page without changing the data for the lower page.

In one embodiment, a system can be set up to perform full sequencewriting if enough data is written to fill up a word line. If not enoughdata is written, then the programming process can program the lower pageprogramming with the data received. When subsequent data is received,the system will then program the upper page. In yet another embodiment,the system can start writing in the mode that programs the lower pageand convert to full sequence programming mode if enough data issubsequently received to fill up an entire (or most of a) word line'smemory cells. More details of such an embodiment are disclosed in U.S.patent application titled “Pipelined Programming of Non-VolatileMemories Using Early Data,” Ser. No. 11/013,125, filed on Dec. 14, 2004,inventors Sergy Anatolievich Gorobets and Yan Li, incorporated herein byreference in its entirety.

FIG. 15 is a flow chart describing one embodiment for reading data fromnon-volatile memory cells. The discussion above with respect to thesense modules discusses how data is read from particular bit lines. FIG.15 provides the read process at the system level. In step 800, a requestto read data is received from the host, the controller, or anotherentity. As discussed above, shifts in the apparent charge stored on afloating gate (or other charge storing element) of a non-volatile memorycell can occur because of the coupling of an electric field based on thecharge stored in adjacent floating gates (or other adjacent chargestoring elements). To compensate for this coupling, the read process fora given memory cell will take into account the programmed state of anadjacent memory cell. Step 802 includes determine whether to providesuch compensation for coupling between neighboring floating gates. Insome embodiments, step 802 also includes determining how muchcompensation to use. In step 804, a read process is performed for aparticular page or other unit of data in response to the request to readdata. The read process of step 804 may include appropriate compensationfor coupling between neighboring floating gates, based on step 802. Inone embodiment, the memory cells read in step 804 are connected to acommon word line, but different bit lines.

In one embodiment, when data for a page is programmed, the system willalso create Error Correction Codes (ECCs) and write those ECCs with thepage of data. ECC technologies are well known in the art. The ECCprocess used can include any suitable ECC process known in the art. Whenreading data from a page (or other unit of data), the ECCs will be usedto determine whether there are any errors in the data (step 806). TheECC process can be performed by the controller, the state machine orelsewhere in the system. If there are no errors in the data, the data isreported to the user at step 808. If an error is found at step 806, itis determined whether the error is correctable (step 810). Various ECCmethods have the ability to correct a predetermined number of errors ina set of data. If the ECC process can correct the data, then the ECCprocess is used to correct that data in step 812 and the data, ascorrected, is reported to the user in step 814. If the data is notcorrectable by the ECC process (step 810), an error will be reported tothe user in step 820. In some embodiments, step 820 can also includereporting all or a subset of the data. If it is known that a subset ofdata does not have an error, that subset can be reported.

FIG. 16 is a flow chart describing another embodiment for reading datafrom non-volatile memory cells that potentially uses compensation forcoupling between neighboring floating gates. One difference between theprocess of FIG. 15 and the process of FIG. 16, is that the process ofFIG. 16 uses the compensation only if there is an error during the readprocess.

In step 840 of FIG. 16, a request to read data is received from thehost, the controller, or another entity. In step 842, a read process isperformed for a particular page or other unit of data in response to therequest to read data. The read process of step 842 does not include thecompensation for coupling described herein. Step 844 includesdetermining whether there are any errors in the data. If there are noerrors in the data, the data is reported to the user at step 846. If anerror is found at step 844, it is determined whether the error iscorrectable in step 850. Various ECC methods have the ability to correcta predetermined number of errors in a set of data. If the ECC processcan correct the data, then the ECC process is used to correct that datain step 852 and the data, as corrected, is reported to the user in step854. If the data is not correctable by the ECC process (step 850), thenthe system will attempt to recover the data by performing a read processwith compensation for coupling between neighboring floating gates. Thus,in step 860, the system determines whether and/or how much compensationto use to address coupling between neighboring floating gates. In step862, a read process is performed for a particular page or other unit ofdata in response to the request to read data. The read process of step862 attempts to recover data by using appropriate compensation forcoupling between neighboring floating gates, based on step 860.

A neighboring floating gate to a target floating gate may includeneighboring floating gates that are on the same bit line but differentword line, neighboring floating gates on the same word line butdifferent bit line, or floating gates that are across from the targetfloating gate because they are on both a neighboring bit line andneighboring word line. In one embodiment, the compensation for couplingdiscussed herein can apply to any of these above mention sets ofneighboring floating gates. In some embodiments, the compensation forcoupling discussed herein applies to neighboring floating gates on thesame word line but different bit line. For example, memory cell 362 mayhave its apparent threshold voltage changed due to coupling from memorycells 364 and 366 (see FIG. 7). Compensation for coupling due toneighboring floating gates on the same bit line but different word lineis discussed in more detail in U.S. patent application Ser. No.11/099,049, “Read Operation For Non-Volatile Storage That IncludesCompensation for Coupling,” filed on Apr. 5, 2005, inventors Yan Li andJian Chen, incorporated herein by reference in its entirety. Someembodiments provide compensation for coupling between neighboringfloating gates on the same word line but different bit line and betweenneighboring floating gates on the same bit line but different word line.

The amount of coupling between neighboring floating gates depends on thetiming of when the neighboring floating gates are programmed. Twoneighboring floating gates programmed at the same time are likely tohave little or no coupling. The greatest amount of coupling is likely tohappen between two neighboring floating gates where one floating gate isnot programmed (e.g., remains in erased state E) and the other floatinggate is subsequently programmed to the highest (e.g., most) programmedstate (e.g., programmed to state C—see FIG. 13). Because there is alarge margin between state E and state A, even with coupling there isnot likely to be an error reading data in state E. The second largestamount of coupling is between a first floating gate programmed to stateA and a later programmed floating gate programmed to state C. Thus, inone embodiment, the only time compensation for coupling will be used iswhen a memory cell is in the first programmed state (e.g., state A) andits neighbor is in the highest programmed state (e.g., state C) of a setof states (4 state, 8 states or a different number of states). In otherembodiments, compensation for coupling can be used when a neighbormemory cell is in a different state, such as state B or another state.In some embodiments that use more or less than four states, compensationfor coupling can be used when a neighbor memory cell is in a state thathas been found to cause coupling. Similarly, compensation for couplingcan be used when a target memory cell is in a state other than state A,as appropriate for the particular implementation.

If the neighbor memory cell's state can be detected or acquired usingsome technique, the amount of correction needed for the memory cellunder consideration can be determined and adjusted in the next readoperation. One way to know the state of a particular memory cell isthrough a read operation. But, in one embodiment, there is nocommunication between sense amplifiers for neighboring bit lines. Evenafter a read operation, the memory cell under consideration will notknow the state of its neighbor memory cells.

FIG. 17 is a flow chart describing one embodiment for determiningwhether and how much compensation should be used based on whether amemory cell is in state A and one or more of its neighbors are in stateC. There are at least two scenarios. In one scenario, the particularmemory cell being read is in state A and one of its neighbors are instate C. In the second scenario, the particular memory cell being readis in state A and two of its neighbors (different bit lines) are instate C. The process of FIG. 17 determines whether any neighbors for aparticular memory cell (or particular bit line) are in state C (or in athe highest programmed state of a set of 7 or more states). This processcan be used to implement step 802 of FIG. 15 and step 860 of FIG. 16.

In step 900 of FIG. 17, all of the memory cells (or a subset) connectedto the selected word line are read to determine whether the memory cellsare in state C. This is accomplished by using read compare point Vrc.Those memory cells having a threshold voltage greater than Vrc areassumed to be in state C. Those memory cells having a threshold voltagethat is less than Vrc are not in state C. At the end of the readoperation using Vrc, each sense amplifier will latch whether thatcorresponding memory cell is in state C or not. One hurdle that must beovercome is that in some implementations sense amplifiers cannot talk toneighboring sense amplifiers. Thus, looking at FIG. 7, the senseamplifier for bit line BL2 cannot communicate with the sense amplifierbit line BL1 or bit line BL3. Therefore, the sense amplifier for BL2does not know whether the neighbor memory cells on BL1 and BL3 are instate C. Steps 902-910 are performed to indicate whether neighbor memorycells are in state C. At step 902, all of the bit lines that areconnected to a memory cell sensed in step 900 to be in state C arecharged to a predetermined voltage. In one example, the bit lines withmemory cell in state C are charged to 0.5 volts. Looking back at FIG. 9,this can be accomplished by applying 0.5 volts+Vth (of transistor 612)to the gate of transistor 612 and toggling the RST signal low to causeINV=0. Other sense amplifiers were set up with INV=1, and therefore,their bit lines will not be charged up. With GRS=0, there is no activepull down on the bit line. When the bit lines with C data charge up, theneighboring bit lines will be coupled to those bit lines due to bit lineto bit line coupling. In one implementation, such coupling can be 40% ofthe total bit line capacitance. For bit lines that have both neighborswith C data, capacitive coupling could be up to 80% of the total bitline capacitance. For example, if the bit line has one neighbor with Cdata, it can be coupled-up by approximately 0.15 volts. If the bit linehas two neighbors with C data, it can be coupled-up by approximately 0.3volts.

In step 904, those bit lines having two neighbors with C data areidentified. In one embodiment, step 904 is accomplished by lowering BLCto 0.2+Vth (of transistor 612). This will cause the bit lines with two Cneighbors to have its transistor 612 turn off, since the drain site oftransistor 612 is Vdd and the source side is 0.3 volts. Then, SEN nodewill not be discharged, the sense amplifier will latch LAT=1. Other bitlines with one C neighbor or no C neighbor will have transistor 612conducting. Since the bit line has a much higher capacitance thancapacitor Csa, the SEN node will discharge and the sense amplifier willlatch LAT=0. The result of whether the SEN node charged or dischargedwill be stored in appropriate data latches 394 (step 906). After step906, the sense amplifier and bit lines are reset and those bit linesconnected to memory cells in state C are then charged up again in step908, similar to step 902. In step 910, those bit lines that are coupledto one or more C neighbors are sensed by applying BLC=0.15 volts+Vth(transistor 612). The system senses those bit lines with one or moreneighbors having memory cells in state C. In step 912, the results willbe stored in one of the latches 394. For those bit lines that storedindication that two neighbors are in state C in step 904 and one or moreneighbors are in state C in step 908, it is assumed that that bit linehas two or more neighbors in state C. For those bit lines that did notstore an indication in step 906 that two or more neighbors are in stateC but did store an indication that one or more neighbors are in state Cin step 910, it is assumed that those bit lines have one neighbor instate C.

FIG. 18 is a timing diagram that graphically depicts some of theoperations performed in the process of FIG. 17. The timing diagram isbroken up into three time periods corresponding to steps 902, 904 and906. During step 902, it is seen that the signal BLC is raised to 0.5volts plus the threshold voltage for transistor 612. This is done forall bit lines that are coupled to a memory cell connected to theselected word line and in state C. Those bit lines are shown to beraised to 0.5 volts. Bit lines then with two C neighbors are coupled tothe two corresponding neighbor bit lines such that the bit lines areraised to 0.3 volts. A bit line with no C neighbor will stay at 0 volts.During this timeframe the signal GRS is low. BLC is then lowered down to0 volts and subsequently raised to 0.2 volts plus the threshold voltagefor transistor 612, at which time the bit lines are sensed (step 904).Those bit lines next to two state C neighbors will not discharge the SENnode (see line 914). Those bit lines that did not have two state Cneighbors will discharge the SEN node (see line 916). After latching thedata in the appropriate bit line latch 382, the data will be transferredto the data latches 394.

FIG. 19 describes one embodiment of a read process which can includeproviding compensation for memory cells having one or more C neighbors.The process of FIG. 19 provides more details of one embodiment of step804 of FIG. 15 and step 862 of FIG. 16. Additionally, steps 940-950 and964-972 can be used to implement step 842 of FIG. 16. The process ofFIG. 19 can be performed for a page of data that encompasses a word lineand all bit lines, or a subset of bit lines. In step 940 of FIG. 19,read reference voltage Vra is applied to the appropriate word lineassociated with the page. This causes the read reference voltage Vra tobe applied to the control gates for the memory cells connected to thatword line. In step 842, the bit lines associated with the page aresensed to determine whether the addressed memory cells conduct or do notconduct based on the application of Vra to their control gates. Bitlines that conduct indicate that the memory cells turned on; therefore,the threshold voltages of those memory cells are below Vra (e.g., instate E). In step 944, the results of the sensing for the bit line isstored in the appropriate latches for those bit lines.

In step 946, read reference voltage Vrb is applied to the word linesassociated with the page being read. In step 948, the bit lines aresensed as described above. In step 950, results are stored in theappropriate latches for those bit lines connected to memory cells in thepage that do not have a neighbor memory cell in state C.

This embodiment attempts to correct for data in state A that is next tomemory cells in state C. The error that can be caused is that the memorycells will have an apparent threshold voltage that is increased so thatit appears to be in state B, when it is actually in state A. In step952, Vrb plus a first offset is applied to the word lines associatedwith the page being read. In line 954, bit lines are sensed as describedabove. In step 956, the results are stored in the appropriate latchesfor those bit lines connected to memory cells in the page that have oneneighboring memory cell in state C. In step 958, Vrb plus a secondoffset is applied to the word line associated with the page being read.In step 960, the bit lines are sensed as described above. In step 962,the results are stored in the appropriate latches for those bit linesconnected to memory cells in the page having two neighboring memorycells in state C.

In step 964, read reference voltage Vrc is applied to the word lineassociated with the page being read. In step 966, the bit lines aresensed as described above. In step 968, the results are stored in theappropriate latches for all the bit lines. In step 970, the data valuefor each memory cell in the page (or other unit of data) is determined.For example, if a memory cell conducts at Vra then the memory cell is instate E. If the memory cell conducts at Vrb (or Vrb plus first offset orVrbt plus second offset) and Vrc, but not at Vra, then the memory cellis in state A. If the memory cell conducts at Vrc but not at Vra or Vrb(or Vrb plus either offset), then the memory cell is in state B. If thememory cell does not conduct at Vra, Vrb (or Vrb plus either offset) orVrc, then the memory cell is in state C. In one embodiment, data valuesare determined by processor 392. In step 972, processor 392 will storethe determined data values in the appropriate latches for each bit line.In other embodiments, sensing the various levels (Vra, Vrb, and Vrc) mayoccur in different orders.

The amount of the first offset and second offset is dependent on theparticular implementation. The invention described herein does notdepend on any particular value for the first offset or second offset. Inone embodiment, the first offset is 0.1 volts and the second offset is0.2 volts; however, other values as appropriate may also be used.

Instead of correcting for the capacitive coupling between neighboringmemory cells on different bit lines during the read process, thecompensation can also be performed at the time of programming. Since thesystem will know the data at the time of programming, the system canintentionally program memory cells to state A with a slightly lowerthreshold voltage if that memory cell has one or more of its neighborsassigned to be programmed to state C. This way that state A memory cellwill be read correctly after the neighbor assigned to state C hasfinished programming.

One solution for achieving tight threshold voltage distributions withoutunreasonably slowing down the programming process is to use a two phaseprogramming process. The first phase, a coarse programming phase,includes attempts to raise the threshold voltage in a faster manner andpaying relatively less attention to achieving a tight threshold voltagedistribution. The second phase, a fine programming phase, attempts toraise a threshold voltage in a slower manner in order to reach thetarget threshold voltage while achieving a tighter threshold voltagedistribution. An example of coarse/fine programming methodology can befound in U.S. Pat. No. 6,888,758 incorporated herein by reference in itsentirety.

In one example of a coarse/fine programming method, the process uses twoverify levels: a target verify level (also called a fine verify level)and a coarse verify level. The process will start by performing thecourse phase of the programming process. When the threshold voltage ofthe memory cell reaches the coarse verify level, which is below thetarget verify level, the memory cell will enter the fine programmingphase by raising the bit line voltage to a value greater than 0 voltsand less than the inhibit voltage. During the coarse phase, the bit linevoltage will be at approximately 0 volts. To inhibit a memory cell fromprogramming, the bit line voltage is raised to the inhibit voltage(e.g., Vdd). During the fine programming phase, programming is slowed,as compared to the course programming phase, due to the affect that thebit line voltage is raised from 0 volts to an intermediate value.Therefore, the change in threshold voltage per program step is likely tobe smaller during the coarse programming phase. The memory cell willremain in the fine programming phase until the threshold voltage for thememory cell has reached the target threshold voltage. When the thresholdvoltage of the memory cell reaches the target threshold voltage, the bitline voltage is raised to Vdd (or other inhibit voltage) to inhibitfurther programming of that memory cell.

The proposed method for programming that includes correcting for thecoupling between neighboring memory cells on different bit lines willuse the above-described coarse/fine programming process; however, threeverify levels will be used instead of two. For example, FIG. 20 showsthreshold voltage distribution 980 for state A. The target voltage usedfor verification is Vva. The prior art method for coarse/fineprogramming discussed above has a coarse verification level noted asVca. The proposed scheme includes adding a third verification level Viato be used as discussed below. In summary, during the coarse programmingphase, memory cells will be programmed until the threshold voltagereaches Vca. Those memory cells that require compensation because theyare being programmed to state A and are next to memory cells that are tobe programmed to state C, will be programmed in the fine phase until thethreshold voltage reaches Via. Other memory cells will be programmed inthe fine phase until their threshold voltage reaches Vva. Thus, memorycells in state A that have neighbors in state C are likely to have lowerthreshold voltages, possibly even below target threshold voltagedistribution 980. The coupling, therefore, will cause the thresholdvoltages of those memory cells to be raised into threshold voltagedistribution 980.

FIG. 21 provides graphs of a threshold voltage versus time and bit linevoltage versus time to indicate one example of coarse/fine programmingfor memory cell that did not need compensation because neither itsneighbors are in state C. The graphs assume that at times t1, t2, t3, t4and t5 a programming pulse is applied to the control gate for the memorycell. At the pulses associated with t1, t2 and t3, the threshold voltageof the memory cell is increased. At time t3 the memory cell's thresholdvoltage becomes higher than Vca. Therefore, the coarse programming phaseis over and the fine programming phase begins. The bit line voltage israised accordingly from 0 volts to intermediate voltage V1 (e.g., onevolt). The application of the intermediate voltage V1, as opposed to 0volts, slows down the programming process for that bit line. At time t5,when the threshold voltage of the memory cell is greater than Vva, thebit line voltage will be raised to be inhibit voltage (e.g., Vdd).

FIG. 22 shows graphs for a memory cell that does require compensationbecause one or more of its neighbors are in state C and this memory cellis being programmed to state A. At time t3, the threshold voltage ofmemory cell has increased to reach Vca; therefore, the bit line voltageis raised to intermediate voltage V1. At time t4, the threshold voltageof the memory cell reaches Via, which is greater than Vca and less thanVva; therefore, the memory cell is locked out from further programmingby raising the bit line voltage to Vdd.

Note that in other embodiments, multiple intermediate voltages can beused in addition to V1. For example, memory cells receiving compensationmay use one intermediate bit line voltage and memory cells not receivingcompensation may use another intermediate bit line voltage. In otherembodiments, different bit lines may use different intermediatevoltages.

FIG. 23 is a flowchart describing one embodiment of a process forprogramming according to the graphs of FIGS. 21 and 22. In step 700, a“data load” is issued by the controller and received by the controlcircuitry. In step 1002, the address data designating the page addressis input to decoder 314 from the controller or host. In step 1004, apage of program data (or other unit of data) for the addressed page isinput to a data buffer for programming. That data is latched in theappropriate set of latches. In step 1006, a “program” command is issuedby the controller to state machine 312. In step 1008, a determination ismade regarding whether to compensate for coupling. For examplecontroller 350, control circuitry 310, sense block 400 or anothercomponent will determine if a particular memory cell will need toreceive compensation during the programming process because theparticular memory cell is being programmed to state C and one or more(or two or more) of its neighbors will be programmed to state C. Sincecontroller 350 and control circuitry 310 are aware of all the programdata, in one embodiment, the system will automatically know whethercompensation is necessary. In other embodiments, each of the datalatches for each bit line will know the data to be programmed.Therefore, sense modules 400 can perform steps 908, 910 and 912 of FIG.17 to determine whether any of the bit lines have neighbors with data tobe programmed in state C. If so, those bit lines that have suchneighbors are marked for compensation. In one embodiment of FIG. 23,there is only one compensation value provided to a memory cell with oneor more neighbors in state C. In other embodiments, differentcompensation of values can be provided depending on whether there is oneneighbor in state C or two neighbors in state C.

In step 1010 of FIG. 23, the initial pulse is set to its initial value,the program counter is set to its initial value, and the bit linevoltage is set to its initial value. For memory cells to be programmed,the bit line voltage will be set at 0 volts. For memory cells that willnot be programmed, the bit line will be set at Vdd. An indication of theinitial voltage can also be stored in a latch. In some embodiments, theinitial bit line value can be applied during program pulse step 1012(discussed below).

In step 1012, a program pulse is applied to the appropriate word line.In step 1014, a verification process is performed. If the memory cellsare in the coarse programming phase, then the verification process ofstep 1014 will be used to determine whether the memory cells' thresholdvoltages have reached the coarse verification level. If the memory cellsare in the fine programming phase, the threshold voltage for the memorycell will be compared to either the target threshold voltage (e.g., Vva)or the intermediate verification level (e.g., Via) for those memorycells that require compensation. More details of step 1014 will beprovided below. In step 1016, it is determined whether the status forall the memory cells to be programmed is such that they have all beenverified. If they all have been verified then a successful programmingprocess is reported in step 1018. If they have not all been verified,then in step 1020 the program counter PC is checked against a programlimit value PC max. If the program counter PC is not less than PC max,then the program process has failed and a status of fail is reported instep 1022. If the program counter is less than PC max, then the programvoltage (Vpgm) magnitude is increased by the step size and the programcounter PC is incremented in step 1024. After step 1024, the processloops back to step 1012 to apply the next Vpgm pulse.

FIG. 24 is a flowchart describing one embodiment of the verificationstep 1014 of FIG. 23. In step 1060, the system determines whether thememory cell is in the coarse programming phase or fine programmingphase. Note that the process of FIG. 23 describes the high level processperformed for a group of memory cells (e.g., a page of memory cellsconnected to a common word line). The process of FIG. 24 is performedindividually for each particular memory cell being programmed. In oneembodiment, the sense blocks will be provided with a latch to store anindication of whether a particular memory cell is in the coarse or fineprogramming phase. If the memory cell is in the coarse programming phasethen a verification process will be performed with the coarseverification level (e.g., Vca) in step 1062. That is, the senseamplifier will be used to determine whether the memory cell's thresholdvoltage has reached the appropriate coarse verification level. Forexample, if the memory cell is being programmed to state A, the senseamplifier will test whether the memory cell's threshold voltage hasreached Vca, as described above. If the threshold voltage has reachedthe coarse verification level (step 1064), then the memory cell hascompleted the coarse programming phase. Therefore, at step 1066, the bitline voltage is raised to intermediate voltage V1 so that the memorycell will enter the fine programming phase at the next programmingpulse. After step 1066, the process will continue at step 1080(discussed below) to determine whether the threshold voltage alsoexceeded the fine verification level (or intermediate verification levelis appropriate). If the threshold voltage of the memory cell has notreached a course verification level, then the bit line voltage willremain at the current level in step 1068 so that the memory cell willcontinue the coarse programming phase.

If, in step 1060, it is determined that the memory cell is in the fineprogramming phase, then in step 1080 it is determined whether the memorycell is being programmed to state A and needs compensation for coupling.If not, the verification process is performed in step 1082 using thefine verification level (e.g., the target verification level Vva, Vvb orVvc). If compensation is needed, then in step 1090 a verificationprocess is performed using the intermediate verification level Via. Ifthe threshold voltage of the memory cell is above the appropriateverification level (step 1084), then the memory cell is locked out fromfurther programming in step 1088 by raising the bit line voltage to Vdd.If the threshold voltage of the memory cell is not above theverification level (step 1084), then the bit line voltage is maintainedat its current level in step 1086 and the fine programming phase willcontinue.

The cells floating gate to floating gate coupling effect can becorrected during program sequence, as described above. It can also becorrected during read operations. The following discussion describes aread sequence which incorporates the bitline to bitline coupling effectas a modification factor to the sensing process, so that the read can bemodified according to the neighbor memory cell states. FIGS. 25 and 26explain an embodiment of a process for reading data that allows forproviding compensation to certain memory cells experiencing couplingfrom neighboring memory cells. In step 1100, all bit lines are read todetermine whether the memory cells connected to those bit lines and tothe selected word line are in state C. This is performed by performing aread operation using Vrc as the read compare point. Those bit lineshaving a memory cell in state C will latch an indication that the memorycell is in state C. The read operation is shown in FIG. 18. In step1102, those bit lines with memory cells in states other than state Cwill be charged-up. In one embodiment, those bit lines are charged to0.5 volts. After the bit lines are charged in step 1102, those bit linesthat are connected to a memory cell in state C are charged to between0.25 and 0.4 volts in step 1104. Charging the bit lines connected to amemory in state C in step 1104 will couple up the bit lines charged instep 1102 to a voltage higher than 0.5 volts. For example, FIG. 26 showsbit line BLn representing those bit lines that do not have the memorycell in state C. The graph shows the bit line being charged up duringstep 1102 to 0.5 volts. Bit line BLn+1 is connected to a memory cell instate C and BLn+1 is a neighbor to BLn. During step 1104, bit line BLn+1is charged to some approximately 0.4 volts. Bit line BLn will then becoupled to a higher voltage than 0.5, as depicted by dashed line 1120.Those bit lines not next to a neighbor that were charged in step 1104will remain at 0.5 volts, as depicted by line 1122. In step 1106 of FIG.25, all the bit lines (or a subset of bit lines) will be sensed. The bitlines with a C neighbor will be sensed to have a higher bit linevoltage. Because of the higher the bit line voltage, the bit line willconduct more current which gives the appearance of a lower thresholdvoltage. This will compensate for coupling between neighboring cells.The cells with C neighbors are coupled to a higher floating voltage thanits original program level after their neighbors are programmed. Thisread with floating gate to floating gate coupling compensation willcorrectly read back the original program levels of the cells. This readcorrection is done without the time penalty from multiple readoperations. One read operation obtains the results for memory cells thatneed correction and memory cells which do not need correction.

In one embodiment described above, there can be several sensing strobesas cell source noise is being removed. The process described above withrespect to FIGS. 25 and 26 can be applied during all sending strobes orduring the later sensing strobes. For example, in one embodiment withtwo strobes, the first strobe may not use the process of FIGS. 25 and26, while the second strobe may use the process of FIGS. 25 and 26.

The above description describes processes for compensating for floatinggate coupling during programming and during reading. In someembodiments, compensation can be performed during both programming andreading. In most embodiments, however, compensation will either beperformed during programming or during reading, but not during both. Thedecision whether to perform the compensation during reading or duringprogramming can be made based on the use of the memory system. Forexample, if the memory system is going to be used in a host where datawill be programmed very few times but read many times, it may be betterto compensate during programming. Alternatively, if the host willprogram many times and read very few times, then it is better to performthe compensation during the read process.

In one embodiment, the memory system can be manufactured to includetechnology for performing compensation during the read process andduring the programming process. At some point during or after themanufacturing process, the memory system can be configured so that itwill perform the compensation either during the read process only orduring the programming process only.

FIG. 27 provides a flowchart describing a process for configuring amemory system so that it will perform the compensation either during theread process or during the programming process. In step 1200, the memorysystem is manufactured with the ability to perform compensation duringreading and to perform compensation during programming. This may includemanufacturing the semiconductor wafer. Optionally, step 1200 could alsoinclude packaging the wafer using processes known in the art. Thepackage may or may not have a switch to perform the configurationdescribed above. Technology for adding such a switch connected to astorage element on the integrated circuit is known in the art. In step1202, a flag (compensation flag) that is part of the memory systemmanufactured in step 1200 is set based on the intended use to indicatewhether the compensation should be performed during reading or whetherthe compensation should be performed during programming. The flag can beset during the manufacturing process, after the manufacturing process,during the testing process or while the device is being used. In step1204, while using the device, the system will check the compensationflag. If the compensation flag was set to perform compensation duringreading, then in step 1206 the memory system will provide thecompensation for coupling during the read process. If the compensationflag is set for programming, then the memory will provide compensationfor coupling during the programming process (step 1208).

The flag can be set in step 1202 in many different ways. During themanufacturing or testing process, a ROM fuse can be set to eitherindicate that compensation should be performed during reading or duringprogramming. In other embodiments, other means (e.g., a memory cell inthe non-volatile array, a flip flop, or other storage device) forstoring indication of a flag can be implemented and/or set during themanufacturing process, or later. The flag can also be set during thetesting process or during use. Additionally, the packaging for theintegrated circuit could include a switch that can be set by the userprior to insertion of a memory card into a host.

In some embodiments, the compensation flag can be set in step 1202 afterthe memory system is inserted into the host. FIGS. 28-31 provideexamples of such a configuration. In step 1300 of FIG. 28, the memorysystem is installed in the host. Examples of a host can include adigital camera, music player, mobile telephone, handheld computingdevice, or other computing device. For example purposes, consider that amusic player may read a lot more often than program. Therefore, a musicplayer may provide compensation during programming. On the other hand, adigital camera may program more often, therefore, it may be moreappropriate to provide the compensation process during the read process.In step 1302 of FIG. 28, the host will notify the controller of itspreference. That is, the host will be preprogrammed to know that it cantell the controller using a known protocol when it wishes to perform thecompensation. In step 1304, the controller will receive the preferencefrom the host and set the compensation flag (stored in a memory cell orother storage device) based on that preference received from the host.

FIG. 29 provides a flowchart of another embodiment for configuring amemory system. In step 1320, the memory system is installed in the host.In step 1332, a user can select a preference. In one embodiment, theuser will select a preference by moving a mechanical switch or selectinga preference in a user interface of the host. For example, a user of adigital camera may select to perform compensation during reading and auser of a music playing device may elect to perform compensation duringprogramming. In step 1334, the host notifies the controller of thatpreference. In step 1336, the controller sets the compensation flagbased on the preference received from the host.

FIG. 30 provides a flowchart describing another embodiment of a processfor configuring a memory system. In step 1330, the memory system isinstalled in a host. In step 1332, the controller requests that the hostidentify itself. For example, the host may indicate that it is a digitalcamera, music player, PDA, cell phone, etc. The controller will receivethat information and access a table of host information in step 1334.The table will identify for each model or type of device how to set thecompensation flag. Based on that table and the received information fromthe host, the controller will choose a configuration (e.g., choosewhether to perform compensation during read or program). In step 1336,the controller will accordingly set the flag based on the configurationdetermined in step 1334.

FIG. 31 is a flowchart describing another embodiment of a process forconfiguring a memory system. In step 1360, the memory system will beinstalled in a host. Step 1362, the host will cause multiple files to bestored in the memory system. After a predetermined amount of time, aftera predetermined amount of files have been stored in the memory system orupon command from the host or user, the controller will determine themost representative file type stored on the memory system in step 1364.For example, if ten files are stored and eight of them are music files,the controller will determine that the most representative file is amusic file. At step 1366, the controller will determine theconfiguration based on the representative file type. For example, atable can be stored in the memory system which lists file types and foreach file type a value will be stored for the compensation flag. Thevalue of the flag can indicate whether to perform compensation duringprogramming or reading. In step 1368, the controller will set thecompensation flag based on the configuration determined in step 1366.

The foregoing detailed description of the invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The described embodiments were chosen in order to best explainthe principles of the invention and its practical application, tothereby enable others skilled in the art to best utilize the inventionin various embodiments and with various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A non-volatile storage system, comprising: non-volatile storageelements; control lines in communication with said non-volatile storageelements; and a managing circuit in communication with said non-volatilestorage elements and said control lines, said managing circuit applies acharge to a first set of said control lines associated with a first setof said non-volatile storage elements that are in a first programmedcondition, said managing circuit identifies a second set of saidnon-volatile storage elements that have a neighbor non-volatile storageelement in said first programmed condition by detecting said charge in asecond set of said control lines that are adjacent to said first set ofsaid control lines, said second set of said control lines are associatedwith said second set of said non-volatile storage elements.
 2. Anon-volatile storage system according to claim 1, wherein: said managingcircuit determines that said first set of said non-volatile storageelements are in said first programmed condition prior to said applyingsaid charge.
 3. A non-volatile storage system according to claim 1,wherein: said first programmed condition corresponds to a highestprogrammed state of three or more programmed states and one erasedstate; and said identifying of said second set of said non-volatilestorage elements that have said neighbor non-volatile storage element insaid first programmed condition is only performed on non-volatilestorage elements that are in a first programmed state of said threeprogrammed states and one erased state.
 4. A non-volatile storage systemaccording to claim 1, wherein: said first set of said control lines arebit lines; and said second set of said control lines are bit lines.
 5. Anon-volatile storage system according to claim 1, wherein: said managingcircuit detects said charge in a particular bit line of said second setof said control lines by determining whether a capacitor charges inorder to discharge said particular control line.
 6. A non-volatilestorage system according to claim 1, wherein: said managing circuitreads said second set of said non-volatile storage elements usingcompensation for coupling from said first set of said non-volatilestorage elements.
 7. A non-volatile storage system according to claim 6,further comprising: said managing circuit reads said first set of saidnon-volatile storage elements without using compensation for floatinggate coupling.
 8. A non-volatile storage system according to claim 6,wherein: said compensation includes reading at a read compare level plusan offset.
 9. A non-volatile storage system according to claim 1,wherein: said first set of said non-volatile storage elements and saidsecond set of said non-volatile storage elements are NAND flash memorydevices.
 10. A non-volatile storage system according to claim 1,wherein: said first set of said non-volatile storage elements and saidsecond set of said non-volatile storage elements are multi-state flashmemory devices.